1. Field of the Invention
This invention relates to a liquid crystal display device used for a display unit of information equipment and, more particularly, to a liquid crystal display device for which a wide viewing angle and a high brightness are required.
2. Description of the Related Art
In recent years, liquid crystal display devices of the active matrix type having a thin film transistor (TFT) for each of the pixels have been widely used in a variety of applications owing to their such features as small thickness, reduced weight, operating on low voltages and consuming less electric power. Modern liquid crystal display devices are realizing a large screen with high precision yet maintaining a wide viewing angle, improved brightness and increased contrast. Therefore, there are realized display characteristics comparable to those of a CRT (cathode-ray tube), lending the liquid crystal display device well suited even for such applications as monitors and TV receivers which have, so far, chiefly employed the CRT.
In a liquid crystal display device of the VA (vertically aligned) mode which drives the liquid crystal molecules in a vertically aligned manner, the liquid crystal molecules are tilted in various directions when a voltage is applied unless the alignment film is subjected to the alignment treatment such as rubbing. As a result, alignment regions of different areas are formed on the pixels. In each pixel, further, a boundary line (discrination) of the alignment region is seen as a dark line which is differently arranged for each of the pixels. Therefore, when the display screen is viewed from a tilted direction, there are seen shading, roughness and residual image on the display screen causing the quality of display to be very decreased. As a liquid crystal display device for realizing display characteristics comparable to those of the CRT, therefore, there has been put into practical use a liquid crystal display device realizing a wide viewing angle by utilizing an alignment dividing system such as MVA (multi-domain vertical alignment)(see, for example, Japanese Patent No. 2947350).
A liquid crystal panel of the MVA system has domain regulating means such as protrusions, dents or slits formed in the electrodes or a combination thereof on at least one surface of either one of a pair of substrates. As a liquid crystal material, there are used nematic liquid crystals having a negative dielectric anisotropy. When no voltage is applied, the liquid crystals are such that the liquid crystal molecules are aligned nearly perpendicularly to the substrate. When a voltage is applied, the domain regulating means so works that the azimuths of alignment in which the liquid crystal molecules are tilted are regulated to a plurality of azimuths in each pixel. Polarizer elements are arranged on both sides of the liquid crystal panel such that the axes of absorption are at right angles with each other.
FIGS. 8A and 8B illustrate the constitution of a pixel on a TFT substrate in a conventional liquid crystal display device of the MVA type, FIG. 8A illustrating the structure of a pixel electrode for realizing four-divided alignment and FIG. 8B illustrating the structure of a pixel electrode for realizing upper and lower two-divided alignment. On the TFT substrate as shown in FIG. 8A, there are formed a plurality of gate bus lines 112 extending in the right-and-left direction and nearly in parallel with each other. A plurality of drain bus lines 114 are formed nearly in parallel with each other extending up and down in the drawing and intersecting the gate bus lines 112 via an insulating film that is not shown. Regions surrounded by the plurality of gate bus lines 112 and drain bus lines 114 serve as pixel regions.
Further, a storage capacitor bus line 118 is formed extending nearly in parallel with the gate bus lines 112, and traversing nearly the center of the pixel region.
A TFT 110 is formed near a position where the gate bus line 112 and the drain bus line 114 intersect each other. A drain electrode 122 of the TFT 110 is drawn from the drain bus line 114 so as to be positioned on one end side of an active semiconductor layer formed on the gate bus line 112 and of a channel protection film (both of which are not shown) formed thereon. On the other hand, a source electrode 124 of the TFT 110 is so formed as to be opposed to the drain electrode 122 maintaining a predetermined gap and is positioned on the other end side of the active semiconductor layer and of the channel protection film. A region of the gate bus line 112 just under the channel protection film works as a gate electrode of the TFT 110. Further, the source electrode 124 is electrically connected to the pixel electrode 116 via a contact hole (not shown).
A pixel electrode 116 is formed in the pixel region. Referring to FIG. 8A, the pixel electrode 116 includes trunk portions 128 extending nearly in parallel with, or perpendicularly to, both bus lines 112 and 114, branch portions 130 branching from the trunk portions 128 and extending aslant, and slits 132 among the neighboring branch portions 130. On an opposing substrate stuck to the TFT substrate and facing thereto maintaining a predetermined cell gap, there are formed a transparent electrode (not shown) on the whole surface of the display region including a plurality of pixel regions. In the MVA-LCD fabricated by using the TFT substrate shown in FIG. 8A and the opposing substrate that is not shown, the directions for aligning the liquid crystal molecules are determined by the trunk portions 128 of the pixel electrode 116, branch portions 130 and slits 132.
Liquid crystals having a negative dielectric anisotropy are sealed between the two substrates. Liquid crystal molecules are aligned nearly perpendicularly to the surface of the substrate due to the alignment-regulating force of vertically alignment films (not shown) formed on the opposing surfaces of the two substrates. The branch portions 130 and the slits 132 in FIG. 8A have widths which are both, for example, 3 μm, and the pitches among the branch portions and among the slits are both 6 μm. With the slit structure which is as fine as the above-mentioned degree, the liquid crystal molecules Lc are tilted in the directions in parallel with the directions in which the slits 132 are extending when a voltage is applied thereto. When a predetermined voltage is applied across the transparent electrodes of the two substrates and the liquid crystal molecules Lc start being tilted along the directions in which the slits 132 are extending, the tilted state propagates successively to the liquid crystal molecules Lc, and the liquid crystal molecules Lc are tilted in the same directions among the slits 132.
Thus, upon arranging the slits 132 in the pixel electrode 116, it is allowed to regulate the direction of tilt of the liquid crystal molecules Lc for each of the regions. If the slits 132 are formed in two directions which are nearly perpendicular to each other as shown in FIG. 8A, the liquid crystal molecules are tilted in four directions in each pixel. Since the viewing angle characteristics of the regions are mixed together, a wide viewing angle is obtained by the MVA-LCD in the white display or in the black display. In the MVA-LCD, a contrast ratio of not smaller than 10 is obtained even at an angle of 80 degrees in the up-and-down right-and-left directions from a direction perpendicular to the display screen.
As shown in FIG. 8A, therefore, when the slit electrodes are so formed that the liquid crystal molecules are tilted in the four directions, the alignments of four domains are realized. As shown in FIG. 8B, further, when the slit electrodes are so formed that the liquid crystal molecules are tilted in the two directions, the alignments of two domains are realized.
In the MVA-LCD using the pixel electrode 116 shown in FIGS. 8A and 8B, however, a response time becomes long from the application of a voltage until the propagation of alignment of the liquid crystal molecules Lc is completed. Therefore, there occur in a random fashion singular points in the alignment vector of the liquid crystal molecules Lc on the branch portions 130. Further, the positions where the singular points are formed migrate for each of the pixels or the frames. When the display screen is viewed from a tilted direction, in particular, there are observed shades and roughness on the display screen, causing the display quality to be deteriorated.
Next, described below with reference to FIGS. 8A to 9D is a relationship between the tilting azimuth of liquid crystal molecules Lc and the directions of axes of absorption of the two polarizing elements P and A. Referring to FIGS. 8A and 8B, the directions of axes of absorption of the two polarizing elements P and A are set being tilted by 45° from the azimuth of alignment of the liquid crystal molecules Lc of when they are tilted. FIGS. 9A to 9D illustrate a relationship between the tilting azimuth of the liquid crystal molecules Lc as seen in a direction perpendicular to the substrate surface and the directions of axes of absorption of the two polarizing elements P and A. FIG. 9A illustrates a case of when no voltage is applied where the liquid crystal molecules Lc are aligned perpendicularly to the substrate surface. On the other hand, light that has passed through one polarizing element P passes through the liquid crystals without affected by birefringence of the liquid crystal molecules, but is shut off by the other polarizing element A to exhibit a black display.
When a voltage is applied, the liquid crystal molecules Lc having a negative dielectric anisotropy are tilted with respect to the substrate surface. When a sufficiently large voltage is applied, the liquid crystal molecules Lc become nearly in parallel with the substrate surface. To realize an optimum white display, the azimuth of alignment of the liquid crystal molecules Lc receives regulation relative to the directions of axes of absorption of the polarizing elements P and A.
FIG. 9B illustrates a case where the liquid crystal molecules Lc are tilted in an azimuth to meet in parallel with, or at right angles with, the axes of absorption of the polarizing elements P and A. In this case, like when no voltage is applied, light that has passed through one polarizing element P passes through the liquid crystals without affected by birefringence of the liquid crystal molecules Lc, but is shut off by the other polarizing element A. Therefore, white display is not aligned.
To obtain an optimum white display as shown in FIG. 9C, the azimuth of alignment of the liquid crystal molecules Lc must be 45° with respect to the axes of absorption of the polarizing elements P and A. In this case, linearly polarized beam that has passed through one polarizing element P becomes an elliptically polarized beam being affected by the birefringence of the liquid crystal molecules Lc, producing light that passes through the other polarizing element A. Therefore, white display is aligned.
To obtain a favorable white display with the four-domain-divided MVA-LCD, therefore, the azimuths in which the liquid crystal molecules Lc are to be tilted and aligned when a voltage is applied are regulated to four azimuths shown in FIG. 9D.
Related Art documents are as follows:
JP-A-2000-29010
JP-A-9-211445
Japanese Patent No. 2947350
Papers in the Panel Discussion, Japanese Association of Liquid Crystals, by Iwamoto, Toko, Iimura, PCa02, 2000
With, for example, the four-domain-divided MVA-LCD as described above, it is desired that the azimuths in which the liquid crystal molecules Lc are tilted and aligned, are four azimuths only as shown in FIG. 9D. In practice, however, due to continuity of liquid crystals, there exist liquid crystal molecules Lc that are tilted in the azimuths other than the four azimuths shown in FIG. 9D.
In the MVA-LCD having a 4-domain electrode structure shown in FIG. 8A, for example, the liquid crystal molecules Lc are tilted in four different azimuths due to fine slits 132 which are so formed as to maintain angles of 45° relative to the axes of absorption of the polarizing elements P and A. In the regions of boundaries where the domains are neighboring each other, however, the liquid crystal molecules Lc are forced to be tilted in the azimuths which are in parallel with, or at right angles with, the axes of absorption of the polarizing elements P and A.
Light does not pass through the region where the liquid crystal molecules are tilted in the azimuths in parallel with, or at right angles with, the axes of absorption of the polarizing elements P and A. In the case of the electrode structure shown in FIG. 8A, therefore, a black region forms in a crossing manner on the white display, which is a major cause that decreases the transmission factor.
To tilt the liquid crystal molecules Lc in a predetermined direction, further, it is necessary to form a line-and-space pattern of a fine pitch as well as to form branch portions 130 of the electrode and slits 132 as shown in FIG. 8A. When a split exposure is employed at a step of photolithography to meet an increase in the size of the panel, however, the branch portions 130 and the slits 132 are formed having widths which are slightly different for each of the split regions due to a slight change in the exposure conditions, whereby shading occurs in the brightness on the display screen when an image is displayed on the panel, arousing a problem of a drop in the production yield.